This project will take place in Mexico City, taking advantage of our current Superfund (SF)-supported project. Recent evidence indicates that there is a marked increase in the mobilization of lead from maternal bone stories into circulation during pregnancy and lactation. Furthermore, our SF data and the data of others indicate that this phenomenon carries a significant risk of fetal toxicity in the form of growth (decreased birth weight, head circumference, birth length) and subsequent cognitive development. These findings pose a major public health problem, even among societies with declining lead exposure, given the persistence of pockets of high lead exposure (including some communities living in proximity to hazardous waste) as well as the long residence team of lead in bone (years to decades). As suggested by our SF data and other studies, one possible strategy for suppressing the mobilization of metal bone lead stores during pregnancy is nutritional intervention. We propose to recruit a new sample of women of women in Mexico City who have relatively high bone lead levels and conduct a randomized, double-blinded, placebo-controlled trial of supplements containing calcium as a means of suppressing bone resorption and the resulting mobilization of lead from bone supplements containing calcium as a means of suppressing bone resorption and the resulting mobilization of lead from bone into plasma during pregnancy, and into breast milk during the postpartum period. We will take maternal measurements of (1) pre-pregnancy and postpartum-bone lead using our K-x-ray fluorescence technology; (2) bone resorption (by assaying (1) pre-pregnancy and postpartum bone lead using our K-x-ray fluorescence technology; (2) bone resorption (by assaying N-telopeptide of type I collagen in urine [urinary NTX]), (3) whole blood lead, and (4) plasma lead (using special collection techniques and measured by IDTIMSJ during pre-pregnancy, the first, second, third trimesters and at one and four months postpartum; and (5) breast milk lead levels are at one and four months postpartum. We will measure maternal plasma and breast milk lead levels as these are the most direct sources of fetal and infant lead exposures, and recent research suggests that maternal venous blood levels do not adequately reflect either of these parameters. We will test the hypothesis that supplements will significantly decrease urinary NTX, plasma lead, and breast milk lead levels. We will also explore the relationship plasma lead levels We will also explore the relationship of plasma lead levels to birth anthropometry measures. This research, if successful, may provide a means of preventing secondary toxicity from accumulated lead burdens among women of reproductive age.